The Third Generation Partnership Project (3GPP) has developed standards for a cellular communications system, referred to as Long Term Evolution (LTE). LTE is based on transmission using Orthogonal Frequency Division Multiplexed (OFDM) signals.
An OFDM signal comprises a plurality of subcarriers, each of which is modulated to carry a transmission symbol. Data for transmission is divided into a plurality of parallel streams, one for each subcarrier. The transmission symbol for each subcarrier is generated by modulating the data using a conventional modulation scheme. Advantageously, OFDM simplifies channel equalisation, by adapting the modulation scheme applied to each sub carrier according to the narrowband propagation conditions for that subcarrier.
In cellular systems, such as LTE, base stations provide radio coverage for individual geographical areas known as cells. In fact, a base station typically comprises multiple co-located radio systems, each of which provides coverage for a different geographical area. Since the antenna of one of these radio systems usually has a directional radiation profile, limited to a well-defined angle, the radio systems are considered to provide coverage to a sector.
A mobile station generally communicates with one radio system at a time. Once communication with a radio system is established, which involves the serving radio system allocating resources to the mobile station, the radio system provides the mobile station with data. In OFDM systems, the allocation of resources can include the subcarriers, symbols timings and antennas used to communicate with the mobile station. This allocation, which is made for both uplink and downlink, can be important to optimise the data capacity available for communication between the base station radio system and the mobile station.
As the mobile station moves it can transfer its service between radio systems, in a process known as handover. In some circumstances, the mobile station can communicate with more than one radio system. The mobile station uses signals transmitted by the radio systems to identify the most appropriate one. It is desirable to make most efficient use of the allocations for each mobile station when communicating with base station radio systems.